Method and apparatus for Raman cross-talk mitigation

ABSTRACT

Disclosed are an apparatus and method configured to process video data signals operating on a passive optical network (PON). One example method of operation may include receiving a data signal at an optical distribution network node (ODN) and identifying signal interference in the data signal. The method may also include modifying a shape of the data signal in the electrical domain and transmitting the modified data signal to at least one optical termination unit (ONT).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/527,697 entitled METHOD AND APPARATUS FOR RAMAN CROSS-TALKMITIGATION, filed Jun. 20, 2012, now issued U.S. Pat. No. 8,923,696,issued on Dec. 30, 2014, which in turn claims priority to U.S.provisional patent application Ser. No. 61/557,719 entitled METHOD ANDAPPARATUS FOR RAMAN CROSS-TALK MITIGATION INTO VIDEO, filed Nov. 9,2011, the entire contents of each are herein incorporated by reference.

TECHNICAL FIELD

Example embodiments provide a method and apparatus of reducing theamount of Raman cross-talk occurring on data content channels, such asvideo channels in optical networking systems.

BACKGROUND

Currently, passive optical network (PON) systems continue to delivercontent to homes and offices across the world. Increasing bandwidth anddata content demands have caused newer signaling protocols andcorresponding data speeds to emerge. Interference signaling and signaldegradation remains a known concern in PONS and next generation gigabit(XG) PON-type systems. In one example, Raman cross-talk is believed tooccur from lower wavelengths into higher wavelengths. For instance, aGPON operating at 1490 nm may cause Raman cross-talk into a 1550 nmvideo overlay service.

One known implementation may include the use of GPON payload scramblingand using a lower GPON transmit power (approximately +5 dBm) to achieveacceptable performance at the optical network termination units (ONTs).Raman cross-talk may also occur at higher wavelengths that traverse intolower wavelengths, such as from 1577 nm into 1550 nm. Though thewavelength spacing is close, which in turn, results in a lower Ramancoupling coefficient, the XGPON-1 power spectral density may be reducedsince the data rate is 10 Gbps, which implies less power on a per-Hzbasis. As a result, the +12.5 dBm optical transmitter power level stillresults in video service degradation when following transmission overthe ODN (i.e., 10-20 km of fiber and splitter loss). The optical inputlevel to an ONT is on the order of −12 dBm. Under these conditions theRaman cross-talk is a significant factor in the recoveredcarrier-to-noise ratio (CNR), signal-to-noise ratio (SNR), andmodulation error ratio (MER) for the first few recovered video channels(55 MHz-120 MHz).

The above-noted performance criteria may be reduced to levelsincompatible with network deployment guidelines. In the case of digitalvideo (256 QAM) the bit error rate (BER) may be reduced to unacceptablelevels. Unacceptable performance levels impact video customer service byplacing impairments or complete loss of recovered video service on somechannels. Some known ways to mitigate the Raman cross-talk impact uponthe video data include significantly reducing the XGPON-1 overalltransmit power level, and using pre-emphasis on the lower video channelmodulation applied to the 1550 nm head-end video transmitter.

Reducing the power transmission results in the inability of the XGPON-1service to have the desired link budget or service distance. Modifyingthe transmitters requires modifications to existing deployed PON systemsand re-configuring thousands of 1550 nm optical video transmitters. As aresult, the existing options for reducing Raman cross-talk includeunfeasible service restrictions and/or expense and complex upgradeswhich are commercially unacceptable and may also lead to backwardscompatibility issues with existing deployments.

SUMMARY

One example embodiment may include a method of receiving a data signalat an optical line termination (OLT). The method may further provideidentifying, by a processor, signal interference in the data signal,modifying, by a processor, a shape of the data signal in the electricaldomain, and transmitting, via a transmitter, the modified data signal toat least one optical termination unit (ONT).

Another example embodiment may include an apparatus including a receiverconfigured to receive a data signal and a processor configured toidentify signal interference in the data signal and modify a shape ofthe data signal in the electrical domain. The apparatus may also includea transmitter configured to transmit the modified data signal to atleast one optical termination unit (ONT).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example PON network system according to exampleembodiments.

FIG. 2A illustrates an example power spectral density comparisonaccording to example embodiments.

FIG. 2B illustrates an example power spectral density comparisonemphasizing a lower frequency range according to example embodiments.

FIG. 2C illustrates a filter response according to example embodiments.

FIG. 3 illustrates an example network entity configured to performcertain operations according to example embodiments.

FIG. 4 is a flow diagram of an example method of operation according toan example embodiment.

DETAILED DESCRIPTION

It will be readily understood that the components of the presentembodiments as generally described and illustrated in the figuresherein, may be arranged and designed in a wide variety of differentconfigurations. Thus, the following detailed description of theembodiments of a method, apparatus, and system, as represented in theattached figures, is not intended to limit the scope of the embodimentsas claimed, but is merely representative of selected embodiments.

The features, structures, or characteristics of the described throughoutthis specification may be combined in any suitable manner in one or moreembodiments. For example, the usage of the phrases “exampleembodiments”, “some embodiments”, or other similar language, throughoutthis specification refers to the fact that a particular feature,structure, or characteristic described in connection with the embodimentmay be included in at least one embodiment. Thus, appearances of thephrases “example embodiments”, “in some embodiments”, “in otherembodiments”, or other similar language, throughout this specificationdo not necessarily all refer to the same group of embodiments, and thedescribed features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

In addition, while the term “message” has been used in the descriptionof the example embodiments of the present disclosure, the embodimentsmay be applied to many types of network data, such as, packet, frame,datagram, etc. For example purposes, the term “message” also includespacket, frame, datagram, and any equivalents thereof. Furthermore, whilecertain types of messages and signaling are depicted in exemplaryembodiments, which are not limited to a certain type of message, and theembodiments are not limited to a certain type of signaling.

FIG. 1 illustrates an example PON configuration according to exampleembodiments. Referring to FIG. 1, a passive optical network (PON) 100implementation may include algorithms, hardware and software aimed atoffering efficient and optimized PON and next generation gigabit (XG)PON1-type systems. PON technology provides a point-to-multipointimplementation (central office point to multiple termination points) toprovide bandwidth, content and other telecommunications services. A PONmay include an optical line termination (OLT) 110 located at the centraloffice and multiple units of optical network units (ONTs) 132, 134 and136 at the customer sites (i.e., offices, homes, etc.). The PON may alsoinclude a splitter or optical distribution network (ODN) node 120 thatprovides data content to the various ONTs.

The International Telecommunications Union (ITU) and the Institute ofElectrical and Electronics Engineers (IEEE) have proposed varioussolutions with respect to PONS in years past. A gigabit-capable PON(GPON) standard provides a sizable amount of total bandwidth andbandwidth efficiency, with a fundamental bandwidth size of 2.488 Gbps ofdownstream bandwidth and 1.244 Gbps of upstream bandwidth. The GPONstandard is widely deployed in many geographical regions.

Example embodiments provide a XGPON-1 type N2B (+12.5 dBm (wavelength1577 nm) downstream transmit power level) device and/or algorithm whichutilizes GPON with a 1550 nm video overlay. Raman cross-talk is known tooccur from lower wavelengths into higher wavelengths. For example, in aGPON at 1490 nm Raman cross-talk may be generated into the 1550 nm videooverlay service. Example embodiments may provide adding XGPON-1 serviceon a deployed ODN without impacting the recovered video performancelevel at the subscriber (ONTs). This implementation would allow anetwork operator to reduce installation costs and migrate customers tohigher bandwidth services with less capital investment.

According to one example embodiment, Raman cross-talk may causeincompatibility between existing and new services at specific frequencyspectrum locations and at predictable power levels. By applyingde-emphasis to the electrical modulation signal of a 1577 nm transmitlaser, it is possible to shape the downstream output optical data signalto reduce the Raman cross-talk level at the critical video frequencies.The shaping of the downstream output optical data signal reductionprocedure can be performed so the previously degraded video channels canbe acceptably recovered.

According to another example embodiment, the shaping of the downstreamoutput optical data signal may be performed in the digital domain. Forexample, a binary input data stream may be digitally filtered to createa required pulse shape. The desired output waveform would be convertedto an analog waveform by way of a D/A (digital to analog) converter.Another implementation would be to include an analog filter in-betweenthe input data stream and an optical modulator. The ideal filterfunction would be a limited high-pass function which would reduce thebaseband non-return-to-zero (NRZ) spectrum of a XGPON-1 configurationjust enough to drop the Raman cross-talk low frequency content to anacceptable recovered video CNR, SNR, and AMR levels. These criteriawould be balanced against the reduction in XGPON-1 recovered BERperformance and increased jitter. The shaping would mildly impact the1577 nm XGPON-1 BER while resulting in a significant increase in the1550 nm recovered video performance.

According to one example method of operation, an optical receiver may beconfigured to receive a video signal via the optical distributionnetwork node (ODN) from an optical line termination (OLT). The OLT mayinclude a processor configured to identify signal interference in thedata signal and modify a shape of the data signal in the electricaldomain. The OLT may be configured to transmit the modified data signalto at least one optical termination unit (ONT). In the procedure, theOLT may also be configured to remove Raman cross-talk interference intothe video signal such that the data signal is modified and subsequentlyprovided to a transmitter which includes a 1577 nanometer (nm) lasertransmitter. The OLT may also be configured to digitally filter the datasignal to create a desired pulse shaped signal and convert the pulseshaped signal to an analog waveform via a digital to analog (D/A)converter. Alternatively, the OLT may also filter the data signal by ananalog filter set between the input data stream and an optical modulatorto remove a low frequency Raman cross-talk interference component and toobtain a limited high pass function with a reduced baseband non-returnto zero spectrum of the data signal. The resulting data signal may be apulse-shaped video signal with a removed low frequency Raman cross-talkinterference component in a 1550 nm video channel range.

FIG. 2A illustrates an example of a power spectral density graph (PSD)according to example embodiments. The dotted line represents an unshapedor unaltered non-return to zero (NRZ) PSD of a XGPON-1. The solid line,which is above the dotted line except from 0 to about 1×10^9 Hz,indicates an example of a high pass filtered signal, which in operationwould reduce the first few video channel Raman cross-talk levels. Thisexample graph 200 illustrates how the shaped or filtered signal providesa larger power spectral density over the various frequency ranges. Otherimplementations are possible and results may vary from oneimplementation to another. The only visible difference between theunshaped and the shaped signals is in the lower frequency range 202.

FIG. 2B illustrates an example of a power spectral density graph (PSD)zoomed into the lower frequency range according to example embodiments.Referring to FIG. 2B, the lower frequency range 204 demonstrates a largeamount of signal shaping in the lower frequencies while the unshapedsignal (dotted line) is kept even without any variations. FIG. 2Cillustrates the filter responses for the high-pass filter represented byan alternating dotted and dashed line, the low-pass filter representedby a solid line and the composite/summation of the high-pass filter andthe low-pass filter which is the dotted line. The composite band rejectshaping approach may reduce the Raman crosstalk by 3 dB in a first videochannel.

Raman cross-talk interference with video overlay can occur in variousdifferent situations. For example, if the interfering wavelength issmaller than the wavelength of the video content (i.e., 1550-1560 nm),then the offending or interfering wavelength creates or “pumps”interfering signals into the video content frequency range causingdistortion of the target signal received at the ONTs. If the interferingwavelength(s) are larger than the video content then by contrast thevideo signals will “pump” interfering signals into the interferingwavelength range. This scenario may seem negligible, however, thepumping may deplete the video signals power level. The depletion mayappear as noise injected into the video signal.

The strength of the signals and fiber distance greatly influence theRaman cross-talk interaction and system/video degradation. Generally,worst case long distance GPON transfers occur in the 9-10 km fiberdistance range. Worst case XG-PON1 transfers occur in the 18-20 km fiberdistance range. Typically the biggest impacts from Raman cross-talkoccur on the 1^(st) handful of video channels. The Raman transfersignals are much stronger with GPON data of the video content. Theimpact from video into XG-PON1 data is approximately 500% less than theprevious scenario. Digital data has a lesser SNR than video. Thecross-talk from XG-PON1 into video is also less, however video is moresusceptible to cross-talk.

There are various different approaches to Raman cross-talk reduction invideo. For example, by using a 2nd feeder fiber for video delivery andimplementing 2:n splitters. Also, by applying video signal pre-emphasisand compensating low channel degradation. 10G-TX de-emphasis may also beused to shape or move the energy spectrum. For orthogonalfrequency-division multiplexing (OFDM), interfering low-frequencysubcarriers can be blanked.

Raman cross-talk could limit co-existence of XGPON-1 in existingdeployed PON networks with video overlay, when high performance (i.e.,low internal noise) video ONTs are deployed, resulting in degradation ofCNR, SNR and MER. An implementation may include altering the data pulseshaping in the electrical domain and applying the pulse into a 1577 nmlaser transmitter, which reduces the resulting Raman cross-talk inspecific 1550 nm video channels. This implementation may allowcoexistence of XGPON-1 and GPON video equipped services in the same ODNnode, and ensures that acceptable video CNR, SNR, and MER levels resultfor the standardized XGPON-1 transmit power levels (up to +12.5 dBm).The implementation allows control of the power penalty impact upon thedownstream 1577 nm signals of the XGPON-1 10 Gbps data path. As aresult, significant cost and capital savings result by applying this forthe network operator.

The operations of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in acomputer program executed by a processor, or in a combination of thetwo. A computer program may be embodied on a non-transitory computerreadable storage medium. For example, a computer program may reside inrandom access memory (“RAM”), flash memory, read-only memory (“ROM”),erasable programmable read-only memory (“EPROM”), electrically erasableprogrammable read-only memory (“EEPROM”), registers, hard disk, aremovable disk, a compact disk read-only memory (“CD-ROM”), or any otherform of storage medium known in the art.

An exemplary storage medium may be coupled to the processor such thatthe processor may read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anapplication specific integrated circuit (“ASIC”). In the alternative,the processor and the storage medium may reside as discrete components.For example, FIG. 3 illustrates an example network element 300, whichmay represent any of the above-described components of the previousdrawings.

As illustrated in FIG. 3, a memory 310 and a processor 320 may bediscrete components of the network entity 300 that are used to executean application or set of operations. The application may be coded insoftware in a computer language understood by the processor 320, andstored in a computer readable medium, such as, the memory 310. Thecomputer readable medium may be a non-transitory computer readablemedium that includes tangible hardware components in addition tosoftware stored in memory. Furthermore, a software module 330 may beanother discrete entity that is part of the network entity 300, andwhich contains software instructions that may be executed by theprocessor 320. In addition to the above noted components of the networkentity 300, the network entity 300 may also have a transmitter andreceiver pair configured to receive and transmit communication signals(not shown).

FIG. 4 illustrates an example flow diagram according to an exampleembodiment. Referring to FIG. 4, the method may include receiving a datasignal at an optical line termination (OLT), at operation 402 andidentifying, by a processor, signal interference in the data signal, atoperation 404. The method may also include modifying, by a processor, ashape of the data signal in the electrical domain at operation 406 andtransmitting, via a transmitter, the modified data signal to at leastone optical termination unit (ONT) at operation 408.

Although an exemplary embodiment of the system, method, and computerreadable medium of the present embodiments has been illustrated in theaccompanied drawings and described in the foregoing detaileddescription, it will be understood that the embodiments are not limitedto the embodiments disclosed, but is capable of numerous rearrangements,modifications, and substitutions without departing from the spirit orscope of the embodiments as set forth and defined by the followingclaims. For example, the capabilities of the system of FIG. 1 can beperformed by one or more of the modules or components described hereinor in a distributed architecture. For example, all or part of thefunctionality performed by the individual modules, may be performed byone or more of these modules. Further, the functionality describedherein may be performed at various times and in relation to variousevents, internal or external to the modules or components. Also, theinformation sent between various modules can be sent between the modulesvia at least one of: a data network, the Internet, a voice network, anInternet Protocol network, a wireless device, a wired device and/or viaplurality of protocols. Also, the messages sent or received by any ofthe modules may be sent or received directly and/or via one or more ofthe other modules.

While preferred embodiments of the present embodiments have beendescribed, it is to be understood that the embodiments described areillustrative only and the scope of the embodiments is to be definedsolely by the appended claims when considered with a full range ofequivalents and modifications (e.g., protocols, hardware devices,software platforms etc.) thereto.

What is claimed is:
 1. A method, comprising: receiving, via an opticalreceiver, a data signal at an optical line termination (OLT);identifying, by a processor, signal interference in the data signal;filtering the data signal by an analog filter set between an input datastream and an optical modulator to remove a low frequency Ramancross-talk interference component; obtaining a limited high passfunction with a reduced baseband non-return to zero spectrum of the datasignal; modifying, by a processor, a shape of the data signal in theelectrical domain to create a desired pulse shaped signal by removingcross-talk interference; and transmitting, via a transmitter, themodified data signal to at least one optical termination unit (ONT). 2.The method of claim 1, wherein removing the cross-talk comprisesremoving Raman cross-talk interference from the received data signal. 3.The method of claim 1, wherein the modified data signal is provided to a1577 nanometer (nm) laser transmitter.
 4. The method of claim 1, furthercomprising: digitally filtering the data signal to create the desiredpulse shaped signal; and converting the pulse shaped signal to an analogwaveform via a digital to analog (D/A) converter.
 5. The of claim 1,wherein the data signal is a video signal.
 6. The method of claim 1, andthe removed low frequency Raman crosstalk interference component is in a1550 ran video channel.
 7. An apparatus comprising: a receiverconfigured to receive a data signal; a processor configured to identifysignal interference in the data signal, remove cross-talk interference,filter the data signal by an analog filter set between an input datastream and an optical modulator to remove a low frequency Ramancross-talk interference component, obtain a limited high pass functionwith a reduced baseband non-return to zero spectrum of the data signal,and modify a shape of the data signal in the electrical domain to createa desired pulse shaped signal; and a transmitter configured to transmitthe modified data signal to at least one optical termination unit (ONT).8. The apparatus of claim 7, wherein the cross-talk interferencecomprises Raman cross-talk interference.
 9. The apparatus of claim 7,wherein the modified data signal is provided to a 1577 nanometer (nm)laser transmitter.
 10. The apparatus of claim 7, further comprising:digitally filtering the data signal to create the desired pulse shapedsignal; and converting the pulse shaped signal to an analog waveform viaa digital to analog (D/A) converter.
 11. The apparatus of claim 7,wherein the data signal is a video signal.
 12. The method of claim 7,and the removed low frequency Raman crosstalk interference component isin a 1550 ran video channel.
 13. A non-transitory computer readablestorage medium configured to store instructions that when executedcauses a processor to perform: receiving, via an optical receiver, adata signal at an optical distribution network node (ODN); identifying,by a processor, signal interference in the data signal; filtering thedata signal by an analog filter set between an input data stream and anoptical modulator of the ODN to remove a low frequency Raman cross-talkinterference component; obtaining a limited high pass function with areduced baseband non-return to zero spectrum of the data signal;modifying, by a processor, a shape of the data signal in the electricaldomain to create a desired pulse shaped signal by removing cross-talkinterference; and transmitting, via a transmitter, the modified datasignal to at least one optical termination unit (ONT).
 14. Thenon-transitory computer readable storage medium of claim 13, wherein thecross-talk interference is Raman cross-talk interference.
 15. Thenon-transitory computer readable storage medium of claim 13, wherein thedata signal is modified and provided a 1577 nanometer (nm) lasertransmitter.
 16. The non-transitory computer readable storage medium ofclaim 13, wherein the processor is further configured to perform:digitally filtering the data signal to create the desired pulse shapedsignal; and converting the pulse shaped signal to an analog waveform viaa digital to analog (D/A) converter.
 17. The method of claim 1, whereinthe removed low frequency Raman cross-talk interference component is ina 1550 nm video channel associated with the data signal.